feat: basic caldav support (#14372)

fix #13531 

#### PR Dependency Tree


* **PR #14372** 👈

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* CalDAV calendar integration: link and sync CalDAV-compatible calendars
(discovery, listing, event sync).
* New UI flow and dialog to link CalDAV accounts with provider
selection, credentials, and display name.

* **API / Config**
* Server exposes CalDAV provider presets in config and new GraphQL
mutation to link CalDAV accounts.
  * New calendar config section for CalDAV with validation and defaults.

* **Tests**
  * Comprehensive CalDAV integration test suite added.

* **Chores**
* Removed analytics tokens from build configuration and reduced Cloud
E2E test shards.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

tools/utils/src/build-config.ts

@@ -53,8 +53,6 @@ export function getBuildConfig(
         linkPreviewUrl: '/api/worker/link-preview',
         CAPTCHA_SITE_KEY: process.env.CAPTCHA_SITE_KEY ?? '',
         SENTRY_DSN: process.env.SENTRY_DSN ?? '',
-        MIXPANEL_TOKEN: process.env.MIXPANEL_TOKEN ?? '',
-        GA4_MEASUREMENT_ID: process.env.GA4_MEASUREMENT_ID ?? '',
       };
     },
     get beta() {